The present invention relates to laser devices, particularly though not exclusively, semiconductor laser devices such as laser diodes, eg a single mode index guided laser diode.
For many applications semiconductor laser devices are desired to operate with a substantially single spatial mode output. This output is desirable, for example, for increased coupling to single mode fibres, and for generating small spot sizes with high light intensities. Typically semiconductor laser diodes generating single mode outputs use index guided laser structures which have either a ridge or a buried heterostructure waveguide. Such laser diodes comprise, for instance as disclosed in JP-A-1 225 288, a device structure comprising a substrate, lower and upper charge carrier confining layers on said substrate, a ridge extending over a portion of said upper confining layer and laterally confining the optical mode of said laser, whereby a layer of active lasing material is sandwiched between said confining layers, said layer comprising a Quantum Well Intermixing (QWI) structure and being configured as an active region. Further provided are regions of compositionally disordered lasing material laterally bounding said active region, said disordered regions having a larger band-gap energy than said active region.
The compositionally disordered lasing material can be provided by a technique known as Quantum Well Intermixing (QWI) Well Intermixing (QWI). Various QWI techniques exist such as Impurity Induced Layer Disordering, Ion Implantation, Impurity Free Vacancy Disordering, and a Damage Induced Technique.
Though these devices provide a single spatial mode output, the total output power is limited due to the Catastrophic Optical Mirror Damage (COMD) level at the ends (facets) of the laser diode. The laser diode facet is cleaved semiconductor and as such contains a high density of vacancies and broken bonds which can lead to the absorption of generated light. Light absorbed at a laser diode facet generates heat as excited carriers recombine non-radioactively. This heat reduces the semiconductor band-gap energy leading to an increase in absorption so inducing thermal runaway which results in COMD.
Prior art techniques to improve COMD levels and consequently device lifetimes, disclose methods of fabricating Non Absorbing Mirrors (NAM) through the use of re-growth or Impurity Induced Disordering (IID) techniques and passivating the facets by evaporation of sulphur containing compounds or silicon. These methods have the disadvantage of being relatively complex.
An alternative prior art technique is to use a lossy confining waveguide which reduces the intensity on the facets by increasing the propagating optical mode size in the vertical direction or reducing wavelength confinement in the horizontal direction. Lossy confining waveguides have the disadvantage that they are susceptible to fabrication tolerances during manufacture.
It is an object of at least one aspect of the present invention to provide a laser device which obviates or mitigates at least one of the aforementioned disadvantages.
It is a further object of at least one embodiment of at least one aspect of the present invention to provide a semiconductor laser device which has a multiplied output power compared to prior art devices of similar length while retaining a substantially single lobed far field output beam profile.
According to a first aspect of the present invention there is provided a laser device comprising;
at least two lasing regions;
an interference region into which an output of each lasing region is coupled; and
an output region extending from the interference region to an output of the device.
In a preferred and advantageous implementation, the laser device is a semiconductor laser device such as a laser diode.
Preferably the semiconductor laser device is fabricated from a III-V semiconductor materials system such as a Gallium Arsenide (GaAs) based material systems operating in a wavelength range of substantially 600 to 1300 nm, or alternatively an Indium Phosphide (InP) based material system operating in a wavelength range of substantially 1200 to 1700 nm. For example AlGaAs or InGaAsP.
Preferably each lasing region may comprise an optically active waveguide.
Preferably also the lasing regions are arranged substantially parallel to each other.
Preferably at least an input of the output region may comprise an output waveguide, and the output waveguide may be positioned transversely between output ends of the at least two active waveguides. Preferably also the at least two active waveguides may be substantially the same, eg in construction and operation. Preferably there are provided two active waveguides.
Preferably the interference region is a multi-mode interference region, ie a multi-mode interference (MMI) coupler. This arrangement provides a laser device including a multi-mode interference (MMI) coupler.
In relation to the MMI couplers for a 3 dB MMI or 1 to 2 dB MMI, two regimes may operate:
(i) an optical signal injected down a single waveguide of the coupler may be split nominally 50/50 between two waveguides of the coupler with relative phases of a given optical mode in each of the two waveguides being zero; and
(ii) an optical signal injected down the two waveguides will be maximally coupled to the single waveguide when the two waveguides are substantially or effectively identical.
Thus for the present invention these features provide a laser device having an output which has a substantially single lobe in the far field.
Preferably the active waveguides may be current driven to provide optical gain in the laser device. The active waveguides may be ridge or buried heterostructure waveguides. Preferably the active waveguides may be Large Optical Cavity (LOC) waveguides, AntiResonant Reflecting Optical Waveguides (ARROW), Wide Optical Waveguides (WOW), or the like.
Preferably each active waveguide may be at least partly formed by a core layer of active lasing material sandwiched between first and second cladding/confining layers formed on a substrate. More preferably, the active lasing material may comprise or include a Quantum Well Intermixing (QWI) Well (QUANTUM WELL (QW) structure configured as an optically active region.
In a modification the active region may be laterally bounded by regions of compositionally intermixed or disordered lasing material. The disordered regions may have a larger band-gap energy and therefore a lower optical absorption than the active region.
Preferably, each active waveguide may comprise a ridge waveguide having a ridge formed in at least the second cladding layer distal the substrate.
More preferably the disordered regions may be formed by Quantum Well Intermixing (QWI) Well Intermixing (QWI). The QWI washes out the Quantum Well Intermixing (QWI) Well confinement of the wells within the core layer. The QWI may be impurity free. The QWI regions may be xe2x80x9cblue-shiftedxe2x80x9d, that is, typically at least 20 meV or 30 meV and normally 100 meV or more difference exists between the active region which is electrically pumped, in use, and the QWI passive regions which are not electrically pumped.
The output waveguide may be optionally active or passive. The output waveguide may be a ridge or buried heterostructure waveguide. Preferably the output waveguide may be a Large Optical Cavity (LOC) waveguide, AntiResonant Reflecting Optical Waveguide (ARROW), Wide Optical Waveguide (WOW), or the like.
Preferably the output waveguide may comprise the core layer sandwiched between the first and second cladding layers.
In one arrangement, the interference region may be optically active. However, in a preferred arrangement, the interference region may be passive. The interference region may comprise a ridge or buried heterostructure.
Preferably the interference region may comprise the core layer sandwiched between the first and second cladding layers.
Preferably also the device further comprises attenuation means. The attenuation means may comprise an etched pattern. The etched pattern may be a notch, groove or the like. Preferably the attenuation means is located on one or more faces of the interference region adjacent the active or passive waveguides. The attenuation means may be located on a face on either side of the passive waveguide and/or on a face between the active waveguides. The attenuation means may act to limit spurious reflections emanating from face surfaces which would degrade an output beam quality of the device.
Preferably the device further comprises a back end and an output end. The back end may be adjacent the active waveguides. The output end may be adjacent the output waveguide.
Preferably the back end may be back surface which may include a high reflectivity coating. More preferably the back surface may be a non-absorbing mirror (NAM). The NAM may be fabricated so that portions of the NAM at ends of the active waveguides have higher band-gap energies than remaining portions of the active waveguide. The NAMs may be fabricated by the same techniques used for the waveguides, and may be QWI regions.
Preferably the output end may comprise an output coupler so that a portion of optical radiation (light) is reflected back into the output waveguide while remaining optical radiation is output from the device. Preferably the output coupler may be a partial reflector. Alternatively the output coupler may be a NAM as described hereinbefore. Further the output coupler may comprise or include a diffractive waveguide.
Preferably the laser device may be made substantially of a III-V semiconductor material. More preferably, the device is grown on a substrate such that the device is monolithic. The device may comprise first (upper) and second (lower) electrical contact layers which may comprise metallisations. The first contact layer may cover all or parts of the second cladding layer. If the device is of a ridge structure the first contact may cover an upper/outer facing of the ridge. Alternatively the first contact layer may cover at least a portion(s) of the ridge corresponding to the active waveguides only.
According to a second aspect of the present invention there is provided a method of fabricating a laser device the device comprising:
at least two lasing regions;
an interference region into which an output of each lasing region is coupled; and
an output region extending from the interference region to an output of the device;
the method comprising the steps of:
(i) forming, in order:
a first optical cladding/charge carrier confining layer;
a core layer (in which is optionally formed a Quantum Well Intermixing (QWI) structure); and
a second optical cladding/charge carrier confining layer;
(ii) selecting regions of the device to be the lasing regions, interference region, and output region.
Preferably the method includes the further steps of:
(iii) forming the interference region and output region;
(iv) defining waveguide means in the lasing regions, the interference region and the output region.
Preferably the waveguide means comprises a ridge or ridges formed in at least the second cladding layer.
Step (i) may be performed by known growth techniques such as Molecular Beam Epitaxy (MBE) or Metal Organic Chemical Vapour Deposition (MOCVD).
Preferably in step (iii) the interference region and the output region may comprise passive regions formed by a Quantum Well Intermixing (QWI) technique which may preferably comprise generating vacancies in the passive regions, or may alternatively comprise implaning or diffusing ions into the passive regions, and annealing to create regions of compositionally disordered material in the core layer having a larger band-gap than the QW structure.
The passive regions may therefore be formed by Quantum Well Intermixing (QWI) Well Intermixing (QWI).
Preferably step (iv) may be achieved by known etching techniques, eg dry or wet etching.
Preferably the method may begin by providing a substrate onto which is grown the first cladding layer, core layer and second cladding layer, in order.
Preferably, step (iii) may be performed by generating impurity free vacancies and more preferably may use a damage induced technique to achieve Quantum Well Intermixing (QWI).
In a preferred implementation of such a technique, the method may include the steps of:
depositing by use of a diode sputterer and within a substantially Argon atmosphere a dielectric layer such as Silica (SiO2) on at least part of a surface of the semiconductor laser device material so as to introduce point structural defects at least into a portion of the material adjacent the dielectric layer;
optionally depositing by a non-sputtering technique such as Plasma Enhanced Chemical Vapour Deposition (PECVD) a further dielectric layer on at least another part of the surface of the material;
annealing the material thereby transferring Gallium from the material into the dielectric layer. Such a technique is described in co-pending application entitled xe2x80x9cMethod of Manufacturing Optical Devices and Related Improvementsxe2x80x9d also by the present Applicant, and having the same filing date as the present application, the content of which is incorporated herein by reference.
Preferably, the method may include the step of applying one contact layer to a surface of the first cladding layer, or more preferably the substrate, and another contact layer to a surface of the ridge.
In a modification, another layer may be grown on the second cladding layer at least part of which another layer may comprise a portion or portions of the ridge.
Preferably the active waveguides may be optically active regions and the interference region and the output region may be optically passive regions.
In a modification, the method may include, preferably in step (iii), forming regions of compositionally disordered lasing material laterally bounding the lasing regions. These may assist the ridge in confining the optical mode(s) of the device.
Advantageously a further region of compositionally disordered lasing material may be formed adjacent to the output region. This further region may be wider than the output region and may act, in use, as a diffractive region at an output end of the laser device.